Re^2: Seeking clarification on possible bug in regex using \G and /gc

Breaks for me on perl 5.26.

perl -E 'local $_ = "foo"; say "Start" if /\G foo/gcx; say "Mid" if /\
+G .*/gcx; say "End" if /\G \z/gcx'
Start
Mid
[download]

Should instead print

Start
Mid
End
[download]

Here is a more extended item looking at pos as well.

perl -E 'local $_ = "foo"; say "Start" if /\G foo/gcx; say pos; say "M
+id" if /\G .*/gcx; say pos; say "End" if /\G \z/gcx; say "At End" if 
+pos == length'
Start
3
Mid
3
At End
[download]

my @a=qw(random brilliant braindead); print $a[rand(@a)];

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Re^3: Seeking clarification on possible bug in regex using \G and /gc by choroba (Cardinal) on Mar 14, 2018 at 23:52 UTC
> Should instead print Why? Using `perl -Mre=debug` will show you your mistake: `... Matching REx "\G \z" against "" 3 <foo> <> \| 1:GPOS(2) 3 <foo> <> \| 2:EOS(3) 3 <foo> <> \| 3:END(0) Match possible, but length=0 is smaller than requested=1, failing! Match failed` [download] The length=1 request comes from the previously mentioned Repeated Patterns Matching a Zero-length Substring: The higher-level loops preserve an additional state between iterations: whether the last match was zero-length. To break the loop, the following match after a zero-length match is prohibited to have a length of zero. ($q=q:Sq=~/;[c](.)(.)/;chr(-\|\|-\|5+lengthSq)`"S\|oS2"`map{chr \|+ord }map{substrSq`S_+\|`\|}3E\|-\|`7**2-3:)=~y+S\|`+$1,++print+eval$q,q,a, [download]	[reply] [d/l] [select]

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Re^3: Seeking clarification on possible bug in regex using \G and /gc
by choroba (Cardinal) on Mar 14, 2018 at 23:52 UTC

> Should instead print

Why? Using perl -Mre=debug will show you your mistake:

...
Matching REx "\G \z" against ""
   3 <foo> <>                |  1:GPOS(2)
   3 <foo> <>                |  2:EOS(3)
   3 <foo> <>                |  3:END(0)
Match possible, but length=0 is smaller than requested=1, failing!
Match failed
[download]

The length=1 request comes from the previously mentioned Repeated Patterns Matching a Zero-length Substring:

The higher-level loops preserve an additional state between iterations: whether the last match was zero-length. To break the loop, the following match after a zero-length match is prohibited to have a length of zero.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

[reply]
[d/l]
[select]